This invention relates to terrain correlation systems, and in particular, to a terrain correlation system for use in inertial navigation systems. While the invention is described with particular reference to its application in missile control applications, those skilled in the art will recognize the wider applicability of the inventive principles disclosed hereinafter.
As appreciated by those skilled in the art, inertial navigation systems require periodic position updates in order to compensate for drift errors introduced during vehicle travel. There are a number of schemes available for updating inertial navigation system position. However, a number of these systems will not function well at all altitudes. Terrain correlation is one method that is adaptable for work at a number of altitudes.
Terrain correlation uses a reference map of terrain height variations to locate the vehicle position in a local earth reference coordinate system. Terrain correlation takes advantage of the natural variations in the elevation of the earth's surface to provide a unique signature for position correction. These variations are the only available signature that can be measured accurately on an absolute scale, both with a sensor and from reference maps produced from a stereographic data base. Terrain contours are relatively invariant with changes in season, weather and time of day, as compared to other measurable signatures, also known in the art. Terrain contours also are measurable with relatively simple techniques that are not overly sensitive to altitude and attitude errors, reducing the total impact of environmental effects on the system. Since position location is made relative to a localized coordinate system, errors associated with transformations between coordinate systems are removed. Since the accuracy of the position is so good and an extremely low probability of a catastrophic false fix is present, terrain correlation also can be used for arming and fusing functions of a missile weapon delivery system, for example.
Terrain correlation is employed to remove the vehicle position errors created by inertial navigation system drift. Prior to implementation of terrain correlation, the vehicle likely is within a known region defined in the art and for the purpose of this specification as an uncertainty basket. In one illustrative implementation, terrain correlation uses a radar altimeter, for example, as the basis for accurately locating the vehicle within the uncertainty basket. The position fix is accomplished by performing a one-dimensional (line) correlation between a set of measured altitudes and a stored reference map of terrain height variations. Several algorithms are available for comparing the measured to the stored altitudes. The one employed in the embodiment described hereinafter is the mean absolute difference with means removed. A geometric presentation of this correlation process is shown in FIG. 1 of the drawings. A reference map of terrain altitudes is obtained from available source data and stored in computer memory. A second map is a set of contiguous measured altitudes. The length of the shorter of the two maps must be equal to the length over which correlation is being attempted. This is called the integration length of the processes and equal to the number of cells (N) times the width of a cell (D). The longer map must be equal to the sum of the length of the uncertainty basket plus the integration length. The width of the reference map is set equal to the width of the uncertainty basket. The mean absolute difference basically provides a measure of the match between the measured samples and the stored references and reaches a minimum value for the best match. The mean values are removed from both sets of data so that only incremental altitudes are compared. Each set of N adjacent measured samples is compared with each set of reference samples and the best match indicates the actual vehicle location.
The comparison is made by subtracting a measured altitude from a reference altitude for each of N consecutive samples parallel to the flight path, determining the absolute value of each difference, and adding the N consecutive absolute values. The result provides one point in the mean absolute difference matrix. The same N measured altitudes are compared with each row of N reference altitudes parallel to the flight path. This provides one cross-track column in the mean absolute difference matrix. The first measured altitude is removed, a new measured sample is added and the new set of N consecutive samples is compared with each reference row. This is accomplished in real time as the vehicle flies from cell to cell. This process is repeated until the down-track uncertainty basket has been examined, providing a two-dimensional mean absolute difference matrix over the entire uncertainty basket.
In the event that there is a perfect match, each difference is zero and the sum of N samples is zero, so the value of the mean absolute difference at that point is zero. Because of the various sources of error, a perfect zero is never obtained, but the minimum of the mean absolute difference matrix provides an accurate indication of the location of the vehicle. A pictorial description of the process is shown in FIG. 2.
While terrain correlation works well for its intended purposes, two known deficiencies with such systems previously have existed. Terrain correlation has not worked well at high altitudes. When a radar altimeter, for example, operates under such circumstances, the resulting measured terrain profile becomes a filtered or smooth version of the actual terrain profile. Consequently, terrain correlation use heretofore has been restricted to relatively low altitudes. The invention disclosed hereinafter solves the problem of high altitude implementation by employing a transformation of the reference terrain altitude maps. The transformation essentially is a simulation of the radar, antenna, terrain background scatter function and altimeter processing to give simulated values of the reference map based on the height of the vehicle. An alternate solution to this problem is the use of a laser sensor with its inherent narrow beamwidth, which reduces the smoothing problem.
A second known deficiency with prior art terrain correlation systems has been the system's dependency on a straight line vehicle path. The invention disclosed hereinafter is designed so that it can maintain cross-track position of the vehicle and provide the required correlation in a satisfactory time frame.
One of the objects of this invention is to provide a terrain correlation system which can be operated independently of altitude of the vehicle in which it finds application.
Another object of this invention is to provide a terrain correlation system which can establish the position of a vehicle in earth reference coordinates for a curvilinear flight path.
Another object of this invention is to provide a terrain correlation system which establishes vehicle position quickly.
Another object of this invention is to provide a terrain correlation system which is compatible with existing inertial navigation systems.
Another object of this invention is to provide a terrain correlation system which is compatible with existing avionic systems for missile controls.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.